This invention relates to methods for introducing controlled spin in single mode optical fiber so as to reduce polarization mode dispersion (PMD).
It is well known that the so-called xe2x80x9csingle mode fiberxe2x80x9d that is commonly used in communication systems is not purely single mode. Rather, when light at a particular wavelength propagates through a single mode fiber, two modes, with perpendicular polarizations, exist in the fiber. Light that propagates through a single mode fiber can be represented by a linear superposition of these two modes. If the fiber is perfectly circularly symmetric, in geometric and in stress symmetries, the two polarization modes are degenerate. They propagate with the same group velocity and have no time delay difference after traveling the same distance in the fiber. However, an actual fiber is not perfectly circularly symmetric. Any circular asymmetry which alters the propagation constant of the fiber with respect to the two orthogonal modes breaks the degeneracy of the two modes. These asymmetries may be geometric or stress induced. As a result, the two polarization modes propagate with different propagation. constants. The difference between the propagation constants is termed birefringence. The birefringence causes a general polarization state to evolve periodically along the fiber. The length over which the polarization state is reproduced is the fiber beat length, which is inversely proportional to the fiber birefringence. When birefringence is present, the two polarization modes travel at different group velocities. The differential time delay between the two polarization modes is called polarization mode dispersion, or PMD. PMD causes signal distortion which is very harmful for high bit rate systems and analog communication systems.
It is also known that PMD may be reduced if the polarization modes xe2x80x9ccouple.xe2x80x9d Mode coupling occurs when the light energy from one polarization mode couples to the other polarization mode. This occurs if the longitudinal uniformity of the fiber is not perfect. Such longitudinal uniformity can be disrupted by errors in core circularity in preform fabrication or the drawing process, or by bends and twists when the fiber is deployed.
The present invention provides a method and apparatus for spinning fiber about its longitudinal axis, while it is being drawn, to form rotational non-uniformities in the fiber.
It is an object of the present invention to provide a method for reducing PMD.
It is a further object of the present invention to provide a controllable and predictable method and apparatus for spinning fiber during the drawing process.
It is an object of the present invention to provide a method for reducing PMD in a fiber, when it is used, by spinning the fiber in a controllable and predictable manner during the fiber drawing process, so as to cause mode coupling when the resultant fiber is used.
The above objects are achieved by the present invention.